This invention relates to stable aqueous latexes which exhibit improved heat and light stability, and good mechanical properties.
Aqueous dispersions of polymers, which are referred to in the art as latexes, are generally known to be useful, both alone and in various formulations, as coatings and impregnants. A wide variety of latexes of differing homopolymeric and copolymeric composition (such as styrene-butadiene copolymers, acrylic homopolymers and copolymers, vinylidene chloride homopolymers and copolymers, etc.) have been developed having specific chemical and/or mechanical properties for particular end use applications. For example, aqueous interpolymer latexes resulting from the emulsion polymerization of monovinylidene aromatic monomers, such as styrene; diolefins, such as butadiene; and monoethylenically unsaturated carboxylic acids, such as acrylic acid; are known to be particularly useful as film-forming binders for pigments in paper coating applications. See, for example, U.S. Pat. Nos. 3,399,080 and 3,404,116.
Unfortunately, however, in the custom designing of latexes having such specialized combinations of properties, the full range of flexibility which might be thought possible in theory (e.g., on the basis of the wide variety of known, desirable characteristics individually possessed by the numerous known classes and species of addition polymerizable monomeric ingredients) have proven not to be entirely attainable in practice by virtue of complicating factors. Such factors include the incompatibility as between the individual classes and/or species of the known monomeric materials; wide divergency of reactivity as between monomeric materials, and the like.
It is desirable that a latex exhibits substantial stability towards decomposition or discoloration upon exposure to heat and light. For example, styrene/butadiene type latexes do not exhibit good heat and light stability. However, it is desirable to incorporate styrene and butadiene into latexes as such latexes exhibit good elastomeric properties as well as other desirable physical properties such as elongation, tensil and modulus. In addition, it is often desirable to prepare ignition resistant latexes for some applications. Unfortunately, common latexes do not often exhibit substantial heat and light stability toward decomposition and especially toward discoloration.
In recognition of the foregoing difficulties, attempts have been made in the prior art to overcome, or to at least minimize such difficulties. For example, in U.S. Pat. No. 4,002,801, it is taught that certain reactive monomers which are not readily copolymerizable with other monomers can nevertheless be satisfactorily introduced to a latex particle by encapsulating or "overpolymerizing" the desired homopolymer or copolymer with a copolymer with which the desired reactive monomers are possible. Unfortunately, even such an improvement is not without disadvantages or limitations. For example, only certain types of encapsulating polymers are practicable, and there exists an upper limit upon the amount of polymer solids that can be thereby prepared without latex flocculation or coagulation.
As taught in U.S. Pat. No. 4,325,856, aqueous copolymer latexes are prepared comprising colloidally dispersed, substantially spheroidal copolymer particles having a predominantly hydrophobic core portion and a relatively hydrophilic polymeric portion which is preferentially oriented toward the outer surface thereof. Although such latexes provide desirable physical properties of various copolymerizable species, such latexes do not exhibit highly balanced physical properties.
Accordingly, it would be highly desirable to provide improved latexes and a process for making the same, by which polymers introducing desirable combinations of properties to said latexes can be dispersed within one another to yield a desirable balance of mechanical properties.